Couplers of many types and variations have been developed for circuits processing signals at microwave frequencies. U.S. Pat. No. 3,516,024 was issued on June 2, 1970 to Lange for an Interdigitated Strip Line Coupler. This coupler, also described in Lange, "Interdigitated Strip-Line Quadrature Hybrid", MTTS Digest of Technical Papers, Dallas, Texas, May 5-7, 1969, pp. 10-13, has become generally known as a Lange coupler. Since this early work on strip line conductors many variations have been developed. These are variously described in Waterman, Jr., et al., "GaAs Monolithic Lange and Wilkinson Couplers", and Brehm et al., "Monolithic GaAs Lange Coupler at X-Band", both of IEEE Transactions on Electron Devices, Vol. ED-28, No. 2, February 1981, pages 212-216, and pages 217-218, respectively; Tajima et al., "Monolithic Hybrid Quadrature Couplers (Braided Structures)", IEEE GaAs IC Symposium, 1982, pages 154 and 155; Kumar et al., "Monolithic GaAs Interdigitated Couplers", IEEE, 1983, pages 359-362; Kemp et al., "Ultra-Wideband Quadrature Coupler", IEEE Transactions, 1983, pp. 197-199; Shibata et al., "Microstrip Spiral Directional Coupler", IEEE Transactions, 1981, pp. 680-689; Lentz, "Compact Transmission Line Consisting of Interleaved Conductor Strips and Shield Strips", U.S. Pat. No. 3,162,717 issued Dec. 22, 1964; Oh, "Three Conductor Coplanar Serpentine-line Directional Coupler", U.S. Pat. No. 3,332,039 issued Jul. 18, 1967; Presser et al., "High Performance Interdigitated Coupler with Additional Jumper Wire", U.S. Pat. No. 4,636,754 issued Jan. 13, 1987; and Podell et al., "Spiral Hybrid Coupler", U.S. Pat. No. 4,800,345 issued Jan. 24, 1989.
These various forms of interdigitated and strip-line conductors provide coupling with various degrees of success. However, there remains the need for a coupler having the simplicity of a Lange coupler but with reduced size and improved performance.
The present invention provides this with a Lange coupler having lumped impedances coupled to it. More specifically, the present invention provides a quadrature coupler comprising input, direct, coupled and isolated ports. A first strip conductor couples the input and direct ports. A second strip conductor couples the isolated and coupled ports.
Lumped-element capacitance is added to the strip conductors. For example, in the preferred embodiment of the invention, an interport capacitor is added between the input and coupled ports, and another interport capacitor is added between the direct and isolated ports. Also, a capacitor is coupled between an intermediate point of each strip conductor and a reference voltage, such as ground. These added capacitors improve the velocity equalization of the even-and-odd modes of the coupler while foreshortening it by approximately a factor of two.
The performance of this size reduced and improved Lange coupler is apparent in that the direct and coupled ports are closely coupled over a broad microwave frequency band. Further, the return loss and isolation are very low over the same frequency band. Thus, a coupler having effective coupling performance, small size and simple design is provided. These and other advantages and features of the invention are apparent from the preferred embodiment disclosed in the following detailed description and the associated drawings.